There are two ways to buy Odoo Enterprise licenses.
The first is through a partner (which you don't want to do?) and the second option is to buy them from Odoo itself. You can buy the enterprise licenses online through this page: https://www.odoo.com/buy As to customization: if you have somebody inside the company that can code in Odoo they can just create modules and add them to the modules list in Odoo. If you don't have anybody that can code you can contact a partner or Odoo itself to develop it for you. There are plenty of partners that do that.For testing: you can test on the Odoo SaaS for free, for a trial of 30 days to see if everything is as you like. If you want to test locally on your own Odoo instance you should contact Odoo, I believe you are allowed to use the license for both db's if they're specifically allowed from Odoo S.A.